Theres no doubt that smartphones are fantastic in many ways but battery life is not one of them, it is a universal problem. This has lead to a number of 3rd party manufacturers offering “extended capacity” batteries for many smartphones, generally offering 30-50% extra capacity from a battery the same physical size.

There is a lot of controversy surrounding these batteries with many saying that the OEM batteries are as energy dense as possible, if you want more capacity you have to buy a battery that is physically larger. In other words, just because your new battery claims to have an extra 500mAh doesnt mean that it does…

The waters are muddied further by the fact that there are conflicting reports, some people claim a huge improvement, some claim none, or worse.

I wanted a non subjective measure of the real, useable capacity of a battery, with the aim of putting the subject to rest.

I emphasized useable capacity because a smartphone will not fully utilize a batteries maximum capacity for safety reasons. My smartphone, a HTC Desire HD, will shut down when the battery reaches a voltage of ~3.2v although its absolute minimum voltage is closer to 3v. Discharging as low as 3v risks permanant damage to the battery though, hence the 0.2v safety margin.

The Setup

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On the far left I have an ammeter measuring the current, I wanted to make sure that the current didn’t drop appreciably as the voltage from the battery declined.

The load is a fairly large motor, I had to try half a dozen motors of varying size to find one that drew a suitable steady current. In testing on my phone my average current whilst idle was ~15mA  and whilst active ~300mA, I wanted somewhere around 100mA because i think thats a good midpoint; slightly lower than the mean because the phone spends more time idle than active.

This motor draws a steady 112mA and is also very quiet, which was a concern.

Just to the right of the motor is a simple switching circuit made from a transistor and a diode, this was so that when the battery voltage gets down to the lower safe limit the Arduino can remove the load and prevent damage to the battery.

Finally the Arduino on the far right; it serves two purposes. Primarily it is used to monitor the voltage across the battery and the running time of the test, and send that data to my computer.

Its second purpose is to switch the load on and off when appropriate as mentioned before.

The actual battery is off screen here, because im a genius.

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Results

I’ll keep this updated with more results as I gather them

Non-OEM Battery (Rated 1230mAh)

  • Initial voltage – 4260mv
  • End voltage – 3196mv
  • Current drain – 112mA
  • Time Elapsed – 35,125 sec

Total measured capacity – 1,093mAh

This is quite a way short of what the manufacturer claimed but it isn’t a hugely surprising result, there was still charge left in the battery at the end of the test, but that charge could not be used by your handset, the phones firmware will shut down the phone well before it hits the minimum voltage of ~3.0v to prevent damage to the battery.

1,093mAh is the use you would see from this battery if it were in your phone.